A CCD image sensor is used in an image reading apparatus such as a scanner as a solid-state imaging device. Time delay integration (TDI) technique can be adopted for increasing sensitivity of the CCD image sensor. According to the TDI technique, a transfer speed of charges to be transferred between pixel rows and an image scanning speed of a sensor are matched, and charges of a predetermined number of pixel rows are added so that the sensitivity is increased. When charges accumulated in two pixel rows are added for example, the sensitivity becomes twice as much.
In addition, when charges accumulated in two adjacent pixels are further added, the sensitivity is further increased. In this case, the resolution of image reading is lowered by half, but the sensitivity is further increased twice as much.
Various models of scanners from a high-speed scanner to a low-speed scanner are used practically, and various scanning speeds are adopted for the scanners. Since a time length of light incidence entering into each pixel row is shortened as the scanning speed is higher, a CCD image sensor needs to be highly sensitive. On the other hand, in a case where the scanning speed is low, an appropriate contrast may be acquired by decreasing the sensitivity rather than by increasing the sensitivity.
In a known CCD image sensor using the TDI technique, the sensitivity can be changed to become higher, but the sensitivity can not be changed to become lower. Accordingly, in a case where a CCD image sensor adopting the TDI technique for use in a high-speed scanner is used even in a low-speed scanner, in order to address lowering the sensitivity, transfer timing of charges is changed according to the scanning speed. Thus, the timing design is required to be accurate for transfer of charges so that the design load is increased.